Circuit opening reset means for normally closed electric switches



March 5, 1968 s, N ETAL Q 3,372,257 I CIRCUIT OPEN'ING RESET MEANS FOR NORMALLY CLOSED ELECTRIC SWITCHES Filed April 4, 1966 INVENTORS. EDWARD S. ARLIN United States Patent Office 3,372,257 CIRCUIT OPENING RESET MEANS FOR N ORMAL- LY CLOSED ELECTRIC SWITCHES Edward S. Arlin, Norwalk, and Wayne T. Watson, Mansfield, Ohio, assignors to Norwalk Thermostat Company,

Norwalk, Ohio, a corporation of Ohio Filed Apr. 4, 1966, Ser. No. 539,764 Claims. (Cl. 200-122) ABSTRACT OF TIE DISCLOSURE A resettable switch having two mutually engageable contacts, each separately mounted for displacement and a reset element having abutments which engage each contact mount to establish a separation of the contacts as a function of the separation of the abutments during a reset operation. The displacement of the reset element to the reset position displaces the contact mount, which makes the principal transit when the switch is tripped to its latch position while displacing the mount of the cooperating contact from its reset position to insure that separation of the contacts is maintained. When the resetting force is removed from the reset element, that element is retracted so that its abutment for the contact mount making the principal transit is spaced from the mount.

This invention relates to electrical switches and more particularly to switches which are tripped under given conditions and resettable only following elimination of said predetermined conditions.

A number of utilizations of electrical controls are known in which a circuit control is actuated from one stable state to another upon the persistence of a given condition for a predetermined interval. Frequently, the return of the control to its first condition is restricted to a reset operation which is calculated to assure the restoration of safe conditions in the system prior to the reactivation of the controls. For example, gas and oil fired burners utilize safety devices which disable their controls in response to conditions indicative of a malfunction and which require manual resetting. It is desirable that such devices be arranged so that the resetting function is effective only when the conditions which triggered the device no longer prevail. Since such devices when improperly reset are often capable of creating dangerous conditions, it is desirable to positively assure that an improper reset cannot be accomplished. This has required the utilization of multiple interlocking controls and equipment of substantial complexity.

An object of the present invention is to improve resettable electrical switches.

A second object is to simplify resettable electrical switches.

A third object is to interlock the reset functions and control functions of electrical switches to avoid actuation of the controls during a resetting operation.

A fourth object is to ope-rate a reliable time delay switch on very low levels of power.

In accordance with the above objects, one feature of this invention resides in a switch having two movable contact blades, one of which has first and second stable positions separated by a region of instabiilty across which it shifts rapidly. A condition responsive means, such as a member which shifts its position in response to a predetermined temperature rise as derived from a control heater in heat transfer relation thereto, is arranged to act upon the one blade to constrain that blade in only one direction so that it can be driven from one of its stable positions toward the other stable position and into the range of instability from which it will advance free of 3,372,257 Patented Mar. 5, 1%?68 the drive means to the other stable position. The drive does not constrain the motion of the actuated blade in the direction opposed to the actuating direction and thereby avoids improper closing of the switch by a manual resetting operation when it is partially actuated. A manually reciprocated plunger is provided with a first portion which is engageable with the one contact blade and a second portion engageable with the actuated blade. These portions are so spaced on the plunger that the first portion is effective on the one contact blade and its contact to move them clear of the actuated contact blade and its contact when the second portion is effective to move the second blade toward its stable reset position. Thus during the resetting operation the one blade is moved from the position it normally assumes in a manner to avoid engaging its contact with the contact on the reset blade.

Another feature of the invention is the utilization of snap action assemblies having bistable characteristics for the actuated blade of the switch.

A further feature involves integrating the actuating means and a contact carrying blade such that the actuated blade is actuated at its contact. With this arrangement the actuating means is effective upon the second blade only while the second blade is in the closed contact condition. During reset the reset means shifts the unitary actuating means and contact out of range of the other blade throughout the time it is being reset to avoid any false operation of the switch during reset.

The above and additional objects and features of this invention will be more apparent from the following detailed description of illustrative embodiments thereof when read with reference to the accompanying drawings in which:

FIG. 1 is a side elevation of one embodiment of the invention employing magnetic attraction to establish one stable position of a switch blade and showing the tripped condition of the switch;

FIG. 2 is the switch of FIG. 1 as it is being reset;

FIG. 3 is the switch of FIG. 1 in its reset and closed circuit condition;

FIG. 4 is a side elevation in the tripped condition of an embodiment of the invention employing an over center snap action with the actuating means effective at the switch contacts and a thermally compensated snap posi tion;

FIG. 5 is the switch of FIG. 4 as FIG. 6 is the switch of FIG. 4 in its circuit condition;

FIG. 7 is a side elevation in the reset and closed circuit condition of an embodiment of the invention emit is being reset; reset and closed ploying another form of snap action with the actuating means effective at the switch contacts; and

FIG. 8 is a side view of an embodiment of the invention having the actuating means effective upon a snap assembly at a region remote from the switch contacts.

Each of the illustrated embodiments of the invention includes a stack 11 of elements assembled upon a rivet 12 having a head 13. The elements making up the stack are assembled in a generally conventional manner by utilizing insulating bushings 14 which have shanks (not shown) fitting within the apertures (not shown) of the adjacent conductive elements in the stack to electrically insulate those elements from the rivet 12 and each other.

The switch of each embodiment is of the thermal time delay variety wherein a bimetallic blade 15 upon which is mounted an intimate heat transfer relationship an electrical heater 16 is shifted in position by the warping of the bimetal in some predetermined relationship to the current flowing in the heater. This current is fed to the heater over leads 17 and 18 from suitable heater terminals 19 and 21 in stack 11. An insulating washer 22 separates the terminals 19 and 21 which are otherwise isolated from the elements of the stack by the centering shanks (not shown) on adjacent bushings 14. Heater 16 is electrically insulated from bimetal blade to complete the electrical isolation of the heater from the primary circuit of the switch.

Magnet latching is employed to maintain the switch of FIGS. 1, 2 and 3 reset. The switch is tripped by separating the magnetically attracting elements sufficiently so that a mechanical bias exceeds the magnetic attraction and the contact blades separate to break the switchs primary circuit. It is reset by moving the released blade from its second stable position into the range of magnetic attraction of sufficient force to overcome the mechanical bias. During the reset operation the other blade of the switch is displaced to prevent completion of the circuit.

In particular, in FIGS. 1, 2 and 3, the switch comprises a first blade 23 arranged with longitudinal stiffening ribs to concentrate its flexure adjacent stack 11. This blade is further stiffened by a stiffening blade 24. A main terminal 25 is in electrical communication with blade 23 and its switch contact 26 through blade 24. A second blade 27 and its main terminal 28 is located in the stack above blade 23 as viewed in the drawings. Both of blades 23 and 27 are resiliently biased upward in the drawings.

Blade 27 has longitudinal stiffening flanges 29 and carries a contact 31 arranged to engage contact 26 when the switch is reset as shown in FIG. 3. It also carries a magnet 32 for latching it in its reset position by its magnetic attraction to ferromagnetic slug 33 carried on the bimetal blade 15. Slug 33 is adjustably positioned on blade 15 by its supporting threaded shank 34 fitting into threaded bushing 35 swaged in a suitable aperture of blade 15. If it is desired to electrically isolate the bimetal blade from the main circuit of the switch, the magnet 32 can be secured to blade 27 by a non-conductive coupling or bushing 35 can be made non-conductive.

When no heat is developed in heater 16, slug 33 is positioned as shown in FIGS. 1 and 3 such that when the switch has been tripped, as in FIG. 1, the magnet 32 and slug 33 are separated beyond the range of magnetic attraction and when it has been reset blade 27 is magnetically held as shown in FIG. 3. Ambient temperature compensation of the position of slug 33 is provided by the opposed flexure with temperature changes which occurs in bimetal blades 36 and 15. These blades are joined at their overlapped ends as by rivets 37 to provide an effective length of blade 36 from the edge of abutting washer 38 to the rivets 37 which flexes upward at its rivet end with increased temperature. This compensates the downward flexure of the free end of blade 15 with temperature increases over the length from rivet 37 to bushing 35. Thus in the range of ambient temperature changes for which the switch is designed, the slug 33 is maintained at its adjusted latching level.

The switch when reset is maintained in the condition shown in FIG. 3. It is tripped when a predetermined amount of heat is applied to bimetal blade 15 to flex the blade downward and move slug 33 out of the region in which the magnetic attraction between the slug 33 and magnet 32 is sufficient to overcome the spring bias formed in blade 27 which tends to move it upward at its outer end. Current flowing in heater 16 in an amount and over an interval sufficient to develop the flexing temperature releases the magnet from the slug to the position shown in FIG. 1.

A rigid blade 39 adjacent the end of stack 11 supports the reset actuator, an insulating plunger 41 having an end 42 abutting the upper face of blade 23, a shank passing freely through an aperture 43 in blade 27 and a flange 44 engaging the upper face of blade 27 adjacent aperture 43. Plunger 41 is guided for reciprocal motion within sleeve 46 which is swaged through an opening in blade 39. The motion of the plunger is generally perpendicular to the major face of the blades 27 and 23 and in the plane of motion of blade 27 between its latched and tripped positions. The upper shank 45 of plunger 41 is capped by a button 47 adapted to receive finger pressure when the switch is reset manually. Plunger 41 is biased to position by the resilience of blade 23 when manual pressure is relieved.

Resetting of the switch is illustrated in FIG. 2. In resetting, blade 27 is displaced to bring magnet 27 into effective range of slug 33 so that a magnetic latch is established to hold blade 27. The lower face of flange 44 on plunger 41 causes this displacement of blade 27 when button 47 is depressed. Throughout the resetting, contact 31 and 26 are held apart by the spaced relationship maintained between blades 27 and 23 by pl nger 41. This relationship is determined by the spacing of those portions of plunger 41 abutting blades 27 and 23, particularly the lower face of flange 44 and the plunger end 42. Thus depression of plunger 41 moves blade 23 downward as it moves blade 27 to the reset position as shown in FIG. 2. During reset the travel of plunger 41 is restricted by blade 24 so that blade 27 cannot be displaced to magnetically latch if bimetal blade 15 has been displaced from its normal latching position. This avoids a false closing of the circuit through contacts 31 and 26 if resetting conditions have not been properly restored.

In the switch of FIGS. 4, 5 and 6 a number of elements correspond to those of the switch of FIGS, 1, 2 and 3. These elements will be identified by the numbered reference character employed previously and distinguished by a single prime to those numbers. In FIGS. 7 and 8 a similar scheme will be employed with like reference characters double and triple primed respectively for like elements. The elements are assembled in FIG. 4 as they are in FIG. 1 in a stack 11' having insulators 14' electrically separating the rivet 12 and the conductive parts. An ambient compensated, two section, bimetal blade 36' and 15 functions both as the actuator to trip the switch and as a current carrying switch blade from main terminal 51 to a conductive button or contact 52 which is threadedly mounted in a bushing 53 secured in blade 15. In the present construction the direction in which the actuator drives to trip the switch is reversed so that it flexes upward in response to temperature increases imposed by heater 16'. Accordingly, the flexure directions are inverted so that bimetal blade 36 flexes its outer end downward for increases in ambient temperature to compensate for upward flexure of bimetal 15 and to maintain the adjusted position of the working face of button 52.

A snap action assembly 54 supports the actuated contact in the switch of FIG. 4. That assembly is supported from blade 32 by means of standoff 55. It comprises a rigid blade 56 and a flexible blade 57 fixedly secured to blade 56 at one end as by swaging over a shank on contact 58 and adjustably secured at its opposite end to a slide 59 which is shiftable longitudinally of blade 56 to adjust the compression in blade 57. An ear 61 on slide 59 cooperates with an car 62 upset out of blade 56 to provide adjustment by means of screw 63 threaded in a suitable tapped aperture in ear 61.

Rigid blade 56 has an open interior portion extending from ear 61 to the left edge of contact 58 and of a width greater than the width of blade 57. This permits the blade 57 and its mounting 55 to be shifted across the face of blade 56 to provide an over center snap action of the assembly.

The assembly 54 is bistable. That is, it has two stable positions and an intermediate range of instability. The reset position as shown in FIG, 6 illustrates a first position of stability for blade 56 and contact 58 wherein the lines of compression for blade 57 extend from the ends of blade 56 to mounting 55 and are above the line of tension developed on the face of blade 56. The assembly tends to be biased to this position by the form 64. The lines of compression for blade 57 can be transferred to a position below the line of compression along blade 56 to cause the snap action assembly to transfer to a position as shown in FIG. 4. This second stable position is the tripped position for the switch since contacts 52 and 58 are separated and the assembly can be returned to its closed contact reset condition only by a reset manipulation.

Motion of the snap assembly 54 is constrained by stop blade 65 so positioned that the excursion of bimetal blade 15' under the switch tripping conditions carries the tension line of blade 56 across the compression lines of blade 57 to a position above those lines as shown in FIG. 4. The overcenter action will then cause the end of blade 56 carrying contact 58 to pivot on its opposite end around the point of abutment with stop blade 65 until extension 66 engages and abuts flange 44'.

Reset is accomplished as shown in FIG. 5 by depressing plunger 41'. Composite plunger 41', 4'7 differs from those of the switch of FIG. 1 since blade 39' is part of the main circuit of the switch from main terminal 67 and the plunger must therefore be insulated from blade 39'. In order to accomplish this, plungers 41 and 47' are both made of insulating material and are coupled within sleeve 46' so that flange 44' is retracted to blade 39 by means of a retraction spring 68. The spring is maintained in compression between the upper face of sleeve 46', which if desired can be insulated, and the lower face of the button head 69 of the plunger.

Depression of plungers 47 and 41' shank of 41 extending through a suitable opening in extension 66 to carry end 42 into abutting relation with extension 71. The compound bimetal blade 36', 15' is sufficiently flexible and resilient to be displaced downward by depressed end 42' as the lower face of flange 44 engages the upper face of extension 66 and carries the end of assembly 54 carrying contact 58 downward to a position from which it snaps to the reset position. This transition position is achieved by carrying the lines of compressive forces for blade 57 above the line of tensile forces for blade 56.

The travel of assembly 54 is limited by stop 72 on the reentrant end of blade 39. Release of the plunger causes its retraction under the influence of spring 63. Bimetal blade 15 is returned by its resilience to a position defined by its temperature and contact 52 engages contact 58 and lifts it slightly to free extension 66 from stop 72, The switch is now conditioned as shown in FIG. 6.

As the switch of FIG. 1, the switch of FIG. 4 has a reset means which transfers a contact carrying blade, blade 56, from a tripped stable position to a reset stable position while insuring that the switch is held open during resetting. This is accomplished by spacing the lower face of flange 44' from the end 4-2 on reset plunger 41' a distance such that the contact face of button 52 is displaced below the contact face of contact 58 when extension 66 abuts stop 72 at the time the flange 4d carries the snap assembly 54 to its transition position. Thus the travel of contact 58 from the transition position of assembly 54 to its reset limit will not close on contact 52 until the plunger 41 is retracted. The switch is held open through all conditions of reset.

Another form of over center snap action switch having a resetting means which prevents closure of the switch during the reset operation is shown in FIG. 7. As in FIG. 4 this switch is actuated through the main circuit contacts 52" and 58". The driving contact 52" is threaded and is mounted in tapped bushing 53 in bimetal blade 75 for adjustment with respect thereto. Bimetal blade 75 is directly secured in stack 11". Ambient temperature compensation is achieved by shift of the pivots or snap points 76 for the assembly by means of bimetallic blade 77 supporting those pivots on bifurcated ends bent normal to the major face of the blade. Increased temperatures cause blades 75 and 77 to deflect upward. Heater 16 on blade 75 provedes the time delay control of the position of contact 52" relative to the snap point 76 and the cause the lower means for displacing blade 78 supporting contact 58 to an overcenter position.

Snap assembly 79 of FIG. 7 comprises blade 78 having cars 81 on each side which are formed to knife edges forming fulcrums fitting within the sockets at snap point 76. Blade 78. is befurcated to the left of contact 58" so that its legs 82 from Which ears 81 protrude straddle tension spring 83. Spring 83 is coupled to blade 78 at 84 adjacent contact 58". The opposite end of spring 83 is secured at 85 to a blade 86 extending from stack 11".

Spring 83 places the legs 82 of blade 78 in compression to provide an overcenter snap action dependent upon the relative positions of the tension axis of spring 83 and compression lines along legs 83 to snap pivot 76. When snap pivot 76 is above the tension axis of spring 83 blade 78 forces contact 58" against the crown of contact 52". Heat imparted to blade 75 by heater 16 flexes contact 52" upward and displaces contact 58 until the tension axis of spring 83 moves above snap pivot 76. At that time the blade 78 and contact 58" are snapped upward. Blade 78 can be limited in its upward motion by its intersection with flange 44" on the reset plunger 41". In practice, its motion is limited by the engagement of spring 83 with the main body of bimetal blade 77 between the downturned ears in which the snap pivots 76 are formed. Thus the limited bifurcated end of blade 77 restricts the arc of motion of blade 78.

Reset of the switch of FIG. 7 is accomplished in the manner of FIG. 5. Depression of spring biased plunger 47" moves the flange 87 at the lower end of plunger 41" into engagement with blade '75 and flexes it downward. Further displacement of the plunger downward brings flange 44" into engagement with blade 78 and carries that blade downward until the tension axis of spring 83 crosses to the underside of the hinge axis of pivot 76. At this time the over center action causes contact 58" to swing downward until the ends 88 of the legs 82 abut the underside of blade 77. At this limit the contact 58" is positioned to engage the crown of contact 52" when blade 75 is released and plunger 41" is retracted by spring 47".

As in the case of the switch of FIG. 4 the separation of contacts 58" and 5 is maintained during reset even after the snap over of blade 78 to its reset position by the spatial relationship of flanges 44" and 87 on plunger 41" and by the limit of travel of blade 78 imposed by abutting end 88.

Another configuration utilizing the snap assembly of the switch of FIG. 4 and modified to be actuated at its end remote from the contact is shown in FIG. 8. Actuation of the assembly 54" is by means of ambient temperature compensated, compound bimetal blade 36", 15" through non-conductive, screw adjusted actuator pin 91. Current in heater 16 causes deflection of the outer end of blade 15 upward to displace snap assembly 54' so that blade 56 and contact 58" are snapped to the tripped position of the switch and clear contact 52".

Contact 52" is carried by resilient blade 92 which is formed to have an upward bias. This blade provides a retraction force for plunger 41" by the force it imposes on end 42" to maintain the upper face of flange 44" against the lower face of rigid blade 39".

The switch is reset by depressing plunger 41" to deflect blade 92 downward and carry contact 52" out of range of contact 58" when flange 44 carries the contact end of snap assembly 54" over center toward its reset condi tion and contact 58" is snapped to its reset position. Release of the plunger causes its return to the position shown through the resilience of blade 92. This permits contact 52" to close upon reset contact 58" provided the actuator pin 91 has moved out of its tripping range. If that pin has not moved out of tripping range, the assembly 54' and contact 58" will retreat with flange 44" and no closure will be possible between contacts 52' and 58" at any intermediate position of the reset plunger.

In recapitulation, the present invention has been illustrated in a number of forms of thermal delay switches wherein both contacts of the switch are carried on flexible blades. One of the contacts is mounted on a blade having two positions of stability, one reset for closing the switch, the other tripped to open the contacts. A thermal delay is arranged to trip the blade to be actuated in response to a predetermined condition represented by a rise in the temperature of a bimetal blade to cause its flexure. Once the switch is tripped, it cannot be closed during the resetting operation. A resetting plunger has two working areas, one for each switch blade and so spaced that when the area effective for resetting the actuated blade is moved into the resetting region to accomplish a reset, its cooperating area displaces the other flexible blade to carry its contact out of range of the contact on the reset blade. This action is accomplished without constraint on the motion of the actuated blade in the trip direction so that it can readily be tripped during reset if conditions warrant.

Certain aspects of the magnetically latched embodiment of a resettable switch shown in FIGS. 1, 2 and 3 and of the snap action assembly of FIGS. 4, 5, 6 and 8 are the subject of patent applications filed herewith Serial No. 540,059 filed April 4, 1966, for Magnetically Controlled Resettable Switch in the names of -E. S. Arlin and W. T. Watson and Serial No. 539,996 filed April 10, 1966, for Snap Action Assembly in the name of W. T. Watson respectively.

It is to be appreciated that the set forth embodiments are illustrative of the invention and lend themselves to many variation-s without departing from its spirit and scope. For example, other thermally responsive means can be substituted for bimetallic blades, and alternative latch structures are available for the actuated blade including other snap action assemblies. Accordingly, these embodiments are not to be interpreted as limiting the invention.

Having described the invention, we claim:

1. A resettable electric switch comprising a first contact; a second contact; a first contact support assembly for said first contact having a first stable position and a second stable position; a displaceable contact support for said second contact; means resiliently biasing said second contact to a position engaging said first contact when said first contact support is in its first stable position; means to trip said first contact support to said second stable position; a plunger mounted for reciprocation along a path parallel to the plane of motion of said first support between said first and second positions; a first abutment integral with said plunger and engageable with a surface of said first support opposed to that surface adjacent said second support; a second abutment integral with said plunger and engageable with a surface of said second support adjacent said first support; and means to bias said plunger to a position separating said first abutment from said surface of said first support; said first and second abutments reciprocating with said plunge!" and being spaced on said plunger parallel to the path of reciprocation a distance greater than the separation of said first and second supports when said first and second contacts are in engagement, motion of said first contact support in a direction from said second position toward said first position without constraint of said first contact support in a direction from said first position toward said second position; and means coupled to said reset means for displacing said second contact from the position to which it is biased for engagement with said first contact while said reset means is actuated to reset said first contact support.

2. An electrical switch comprising a first resilient blade; a first contact on said first blade; a second blade adjacent said first contact; a second contact on said second blade and engageable with said first contact, said first blade being movably mounted and having a first stable position in which said first contact normally engages said second contact and a second stable position in which said first contact is separated from said second contact; means responsive to a predetermined condition for transferring said first blade from its first position to its second position; a reset element mounted for reciprocation along a path in the plane of movement of said first blade between said first and second positions and generally normal to said blades; a first portion integral with said reset element engageable with said second blade and arranged, when moved in a reset direction away from said first blade, to carry said second blade away from said first blade; and a second portion integral with said reset element engageable with said first blade only when said first blade is separated from said second blade by at least a predetermined distance defined by the separation of said first and second portions of said reset element, said distance being sufiicient to separate said first and second contacts, whereby movement of the reset element along its path of travel over a range wherein said second portion is engaged with said first blade in its first and second stable positions will maintain said second contact spaced from said first contact.

3. A combination according to claim 1 wherein said first contact support is resiliently biased toward said second stable position, and including a trippable latch to maintain said first contact support in said first position.

4. A combination according to claim 1 wherein said first contact support is a snap action assembly.

5. A combination according to claim 1 wherein said means to trip said first contact support is mechanically operative thereon through said second contact.

6. A combination according to claim 1 wherein said displaceable contact support is a resilient blade movable in a plane normal to its major face; said first contact support is movable between its first and second positions in the plane of motion and generally normal to the major face of said resilient blade; and said reset means is movable in the plane of motion and generally normal to the major face of said resilient blade.

7. A combination according to claim 6 wherein said reset means and said displacing means are a unitary flanged plunger, and wherein said first contact support at least partially embraces a cross section of said plunger with clearance thereto and is coupled. for reset by abutment of said flange with its embracing region.

8. A switch comprising a base, a rigid blade extending from said base, a snap action assembly mounted on said blade, a first contact on said snap action assembly, a resilient blade extending from said base, a second contact on said resilient blade and engageable with said first contact, said snap action assembly having a first stable position in which said first contact abuts said second contact and a second stable position in which said first contact is spaced from said second contact, thermally responsive means for causing displacement of said snap action assembly to said second position in response to a rise in temperature, a plunger mounted on said rigid blade for movement generally along a path generally normal to said second blade and parallel to the path of motion of said snapt a-ction assembly, a first abutment integral with said plunger adapted when advanced toward said second blade along said path to engage said second blade and move said second contact away from said first contact and a second abutment integral with said plunger and adapted when advanced along said path to abut and push said snap action assembly from said second position toward said first position without constraining motion of said snap action assembly in the direction from said second position to ward said first position and to move said first contact toward said second contact while maintaining a spacing between said first and second contacts.

9. A combination according to claim 1 wherein said first and second abutments on said plunger are separated by a distance at least equal to a spacing of said first and second contact support assemblies to maintain said first contact separated from said second contact plus the travel of said first contact between said second and said first sta'bile positions.

10. A combination according to claim 4 wherein said snap-action assembly is of the over-center type and has a region of unstabile equilibrium between said first and second stabile positions from which said assembly will snap to its first position and wherein said first and second abutments on said plunger are separated by a distance at least equal to the spacing of said first and second contact support assemblies required to maintain said first contact separated from said second contact plus the travel of said snap-action assembly from said region of unsta bile equilibrium to said first position.

References Cited UNITED STATES PATENTS 5/1956 Roberts et a1. 200--116 1/1965 Grabinski 200-416 XR BERNARD A. GILHEANY, Primary Examiner. H. A. LEWITTER, Assistant Examiner. 

